Excited-state reactions ketones, Norrish type

Photochemical C —H insertion of ketone 1 proceeds by initial photoexcitation to give an excited state that can be usefully considered as a 1,2-diradical. Intramolecular hydrogen atom abstraction then proceeds to give a 1,4- or 1,5-diradical, which can collapse to form the new bond. This approach has been used to construct both four- and ftve-membered rings12 11. Photochemical-ly mediated cyclobutanol formation is known as the Norrish Type II reaction. 

Scheffer et al. provided another unimolecular asymmetric transformation involving the Norrish type II reaction, a well-known excited state process of ketones that is initiated by an intramolecular hydrogen atom transfer from carbon to oxygen through a six-membered transition state (Scheme 5). [19a] An adamantyl ketone derivative 27 was found to crystallize from ethanol in very large prisms in the chiral space group P2 2 2. Upon irradiation of these crystals to approximately 10% conversion, the chiral cyclobutanol derivatives 28 were afforded as the major products in 80% ee. 

Scheffer et al. provided another unimolecular asymmetric transformation involving the Norrish type 2 reaction, a well-known excited state process of ketones that is initiated by an intramolecular hydrogen atom transfer from carbon to... 

In photolysis of ketones CIDNP studies have confirmed that the Norrish type I split occurs predominantly from a phototriplet state (32,38,118), although some of the reactions with aliphatic ketones exhibit polarization involving both the excited singlet and the triplet (47,118) states as well as the postulated exci-plex intermediates (71). An exciplex mechanism has also been postulated in the CIDNP observation of the photolysis of tri-fluoroacetophenone with dimethoxybenzene in acid solutions (117). 

This section deals primarily with the photochemistry of aryl substituents which is initiated by the arene excited state. A novel exception is the photochemistry of the ketones (362) and (363), which is perturbed by the presence of the ground state arene rings. Photolysis of (362) in methanol yields (364) this is presumably derived from Norrish Type I cleavage followed by disproportionation to a ketene which is quenched by methanol. The reaction fails for... 

The Norrish Type II reaction of aliphatic and aromatic ketones in isotropic solvents has been studied in considerable detail (26,43), and several aspects of the reaction depend on the conformational mobility of the excited ketone or the 1,4-biradical intermediates formed by y-hydrogen abstraction. In the case of aromatic ketones for example, the triplet lifetime can provide an indication of the facility with which the proper geometry for hydrogen abstraction can be obtained (29,43), the distribution of fragmentation O-cleavage) and cyclization products obtained depends on the conformations available to the triplet 1,4-biradical intermediate and their relative kinetic behavior prior to intersystem crossing (27-30,43-47), and the total quantum yield for the reaction is a function of both of the above factors. For practical reasons, product ratios are usually the easiest aspect of the reaction to monitor, and this is the approach that has been used most commonly in studies of Norrish II reactivity in ordered media (27-30,45). The pertinent features of the triplet state reaction arc illustrated in Scheme 1 (30). 

Main-chain scission has been shown to occur in polyphenylvinylketone under UV irradiation (366 nm). Norrish type-2 scission due to reaction of the first n-II triplet state of the ketone has been shown to occur [422]. Under 7-irradiation, this polymer, which was expected to crosslink according to the Miller rule, was shown to undergo main-chain fracture with a G value of 0.35. Inhibition of the degradation in the presence of napthalene and diphenyldisulphide demonstrates the participation of radicals and excited n-II triplet states in the radiolysis [423]. 

The ketone group is a useful model because it can be excited selectively in the presence of other groups commonly contained in polymer chains, such as the phenyl rings in polystyrene, and so the locus of excitation is well defined. Furthermore, there is a great deal known about the photochemistry of aromatic and aliphatic ketones, and one can draw on this body of information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits a number of photochemical processes from the same excited state. Thus one has a probe of die effects of the polymer matrix on these processes by determination of the quantum yields. The competing processes include (1) fluorescence (Eq. 26), (2) phosphorescence (Eq. 27), (3) the Norrish type-I reaction (Eq. 28), (4) the Norrish type-II reaction (Eq. 29), (5) photoreduction (Eq. 30), (6) the... 

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